Conventionally, in some hearing aids, frequency characteristics compensation or limitation of a dynamic range is performed with respect to audio input in accordance with the hearing characteristics that are left for a person with hearing disorder in order to improve the intelligibility. For people with a neural disorder, such a hearing aid may not be capable of providing sufficient intelligibility.
On the other hand, in research on recognition of sound signals, it has come to be known that a speech can be recognized to a considerable extent without hearing a complete sound signal, that is, even if a component of a sound signal is subjected to noise by a predetermined method.
According to such a research, a sound signal is divided into four frequency bands (0 to 600, 600 to 1500, 1500 to 2500, 2500 to 4000 Hz), each sound signal is subjected to half-wave rectification and is applied to a 16 Hz lowpass filter so as to obtain an amplitude envelope of each frequency band, and is multiplied by a band noise corresponding to each frequency band, and the thus obtained signals are added to generate a signal. Such a signal is referred to as “Noise-Vocoded Speech Sound”. It has been reported that when people with normal hearing ability heard the Noise-Vocoded Speech Sound, an intelligibility of about 80% was obtained.
However, conventional hearing aids often do not provide an adequate intelligibility to people with neural disorders or some of the people with hearing disorders. It is necessary to provide a hearing aid that has an effect on such people and further allows brain activity to be activated. Furthermore, it is necessary to provide a device for training the hearing ability of people with a neural disorder or some of the people with hearing disorders. Moreover, it is necessary to train people with normal hearing ability to recognize speeches produced by people with a neural disorder.